(a) Technical Field
Embodiments of the present invention relate to an organic light emitting device and a method for manufacturing the same.
(b) Description of the Related Art
Recent trends toward lightweight and thin personal computers and television sets require lightweight and thin display devices, and flat panel displays such as a liquid crystal display (LCD) that satisfy such requirements are increasingly being used in place of conventional cathode ray tubes (CRTs).
However, because the LCD is a passive display device, an additional back-light may be needed as a light source. Also, the LCD may have various problems such as a slow response time and a narrow viewing angle.
Among the flat panel displays, an organic light emitting device (organic light emitting diode display, OLED display) has recently become the most promising as a display device for solving these problems.
The OLED display includes two electrodes and an organic light emitting layer interposed between the two electrodes. One of the two electrodes injects holes and the other electrode injects electrons into the light emitting layer. The injected electrons and holes are combined to form excitons, and the excitons in turn emit light as they release energy.
Because the OLED display is a self-emissive display device, an additional light source is not necessary such that the organic light emitting device has lower power consumption as well as a high response speed, wide viewing angle, and high contrast ratio.
The OLED display may be classified as a passive matrix OLED display and an active matrix OLED display according to driving type. In the active OLED display, an electrode and an emission layer are disposed on a thin film transistor array panel. The thin film transistor array panel includes signal lines, switching thin film transistors connected to the signal lines for controlling data voltages, and driving thin film transistors directing the current to a light-emitting device by applying the transmitted data voltages as gate voltages.
Also, the OLED display may be classified as a bottom emission type and a top emission type according to the direction of emission. In the bottom emission type, the light emitted from the emission layer passes through a thin film transistor array panel to exit the display. In contrast, in the top emission type, the light emitted from the emission layer passes through a common electrode to exit the display. For a high aperture ratio, the top emission type may be used because an aperture ratio of the top emission type is not influenced by the presence of the signal lines and the thin film transistors.
In an active matrix OLED display, high mobility, high stability, and uniformity of the thin film transistor are required. To satisfy these characteristics, a bottom gate structure using a polysilicon as a semiconductor may be considered, but because the process of high temperature crystallization of an amorphous silicon is required, the gate metal is limited to using a material having a high melting point. However, a signal line having low resistivity must be used to prevent, for example, a signal delay in a large-sized OLED display, but because a low resistivity material for signal lines has a low melting point, such material may not be used as the gate metal.
Also, while the OLED display of the top emission type may obtain a high aperture ratio, the common electrode must be made of a transparent conductive material. However, because the transparent conductive material has a high resistivity, a drop in the common voltage may be generated.